The present disclosure relates to optical cavities for optical devices. More particularly, the disclosure relates to optical cavities for semiconductor and/or dielectric optical devices incorporating a focusing reflector.
Semiconductor fabrication techniques have enabled the construction of miniaturized optical devices. Two examples of such devices are semiconductor lasers, e.g., vertical cavity surface emitting lasers (VCSELs), and semiconductor optical filters. Through these techniques, optical devices can be constructed having dimensions on the order of only a few microns. Applications for such devices are many and include optical communications as well as the construction of optical circuits.
Semiconductor lasers and filters comprise optical cavities through which light passes before being emitted from the devices. Such optical cavities normally include highly reflective, flat mirrors positioned at opposed ends of the cavities that reflect light back and forth within the cavity. The cavities often include an air gap positioned between the mirrors and, in the case of semiconductor lasers, a gain medium that increases the intensity of the light.
In early designs, semiconductor lasers and filters were only capable of emitting fixed frequencies of optical radiation. More recent semiconductor lasers and filters have been constructed with displaceable mirrors to provide for frequency tuning. Displacement of a mirror at an end of an optical cavity changes the relative spacing of the mirrors and therefore the length of the cavity. As is known in the art, adjustment of the cavity length alters the frequency at which the laser or filter emits radiation.
Optical cavities formed with flat mirrors present significant disadvantages. For instance, flat mirror optical cavities are highly susceptible to losses due to misalignment of the mirrors. This misalignment can be magnified when one or both of the mirrors is displaced during tuning. In addition, even where the mirrors are aligned correctly, diffraction losses can occur. To reduce such losses, recent semiconductor lasers and filters have been constructed with a concave, semispherical mirror at one end of the optical cavity. With such a configuration, light is reflected back on itself within the device cavity to prevent the light from escaping.
FIGS. 1 and 2 illustrate an example prior art semiconductor laser 100 and filter 200, respectively. As indicated in FIG. 1, the semiconductor laser 100 comprises an optical cavity 102. At one end of the cavity 102 is a first mirror 104 and at the other end of the cavity is a second mirror 106. Below the second mirror 106 is a substrate 108 constructed of a semiconductor material. Formed on the substrate 108 is a first current injection layer 110 that is used to provide current to the laser 100 during operation. Disposed within the optical cavity 102 is an active region 112 that is responsible for generating the light that is emitted out of the laser 100. In contact with the active region 112 is a second current injection layer 114 that, like the first current injection layer 110, is used to provide current to the laser 100. Formed on top of the second current injection layer 114 are support posts 116 that, together with support tethers 118, suspend the first mirror 104 above the active region 112. Normally formed on the support tethers 118 are tuning electrodes 120 that are used to deliver voltage to the first mirror 104 that displaces it when the laser 100 is tuned. As is evident from FIG. 1, the first mirror 104 is arranged in a concave, semispherical orientation such that light incident on the first mirror is focused inwardly on itself to prevent diffraction losses.
FIG. 2 illustrates the semiconductor filter 200. As is apparent from this figure, the semiconductor filter 200 is similar in construction to the semiconductor laser 100 shown in FIG. 1. Accordingly, the filter 200 comprises an optical cavity 202 that is defined by a first mirror 204 and a second mirror 206. In addition, the semiconductor filter 200 includes a substrate 208, first tuning electrode 210, support posts 212, support tethers 214, and second tuning electrodes 216. Accordingly, the semiconductor filter 200 primarily differs from the semiconductor laser 100 of FIG. 1 in the omission of the active region 112.
Although capable of providing for reduced losses, optical cavities having a concave, semispherical mirror are difficult to manufacture. As is known in the art, it is difficult to form a precise concave surface on a very small scale (e.g., 10 xcexcm in diameter) through present semiconductor fabrication techniques. Accordingly, it can be appreciated that it would be desirable to have a tunable, low-loss optical cavity for semiconductor lasers and filters that does not require a concave, semispherical mirror.
The present disclosure relates to an optical cavity, comprising a first non-concave reflector positioned at a first end of the optical cavity and a second non-concave reflector positioned at a second end of the optical cavity that receives and reflects light reflected from the first non-concave reflector. The first non-concave reflector is configured to focus light that reflects off of the reflector back upon itself to avoid diffraction losses from the optical cavity.
In one embodiment of the invention, the first non-concave reflector includes a layer of material that has a thickness that varies as a function of radial distance out from an axial center of the layer. By way of example, the outer layer can include a substantially convex, semispherical outer surface and a substantially planar inner surface.
In another embodiment of the invention, the first non-concave reflector includes a layer of material that has an index of refraction that varies as a function of radial distance out from an axial center of the layer.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.